In traditional investment ("lost wax") casting the first stage is to produce a wax pattern facsimile of the finished metal part; this is usually done via a specially built tool. The wax patterns are then mounted onto a wax "tree" that acts as the gating and runner system of the mould. This construction is then repeatedly coated ("invested") with layers of ceramic material--usually up to about eight coats. After "shelling", the wax has then to be removed to leave a mould in which to pour the molten metal. Dewaxing is normally achieved in a steam autoclave, which essentially works as a large pressure cooker heating the construction to a temperature of about 165.degree. C. at about 7 bar pressure. After wax removal in the autoclave, the "green" ceramic shells are fired to burn out the residue wax and to fire the shells into a "real" ceramic. After firing, metal is then poured into the mould to produce the final metal casting.
Investment casting is a good way to produce complex and intricate metal parts of various dimensions in one single operation. However, the cost of tool production to produce the wax patterns is so expensive as to preclude its use for anything but medium to large batch production casting. Prototyping parts by this manner is not cost effective.
In stereolithography, electromagnetic radiation is used to selectively solidify a thermosetting liquid material. The solidification takes place in a number of passes across the material to usually produce a solid structure. Generally a plurality of levels are built up comprising plates of set material, which by varying the curing pattern in adjacent levels can also provide a honeycomb type structure.
The electromagnetic radiation is generally in the form of a laser which moves relatively slowly over the liquid when setting is required but skips over areas where setting is not required. Each level is made by a number of passes of the laser over the liquid material.
The laser usually, but not always, acts on the surface of the resin and cures material at this point to a small depth. With most stereolithography systems, the layer which represents the bottom of the part (as it is building) is cured first. This is then covered with a fresh layer of resin and this is subsequently cured. The final layer to be cured (as it is building) is at the top of the part. The cured material is usually located on an open support which is gradually lowered into a bath of liquid resin.
The stereolithography process is simply producing a series of two dimensional profiles which, when stacked on top of each other can produce a complex three dimensional object. Therefore it is unlimited in the complexity of parts it can produce.
Stereolithography therefore appears a suitable process for producing patterns for investment casting. However, as the materials used in this process are thermoset materials, these do not melt in the steam autoclave but usually expand upon heating. This expansion will generally cause the relatively weak unfired ceramic shell to crack thereby potentially damaging the mould and also the model.
Attempts have been made to use stereolithography to produce patterns with a thin skin surrounding an open structure. Such attempts however have not always proved particularly successful. The criteria required of such a pattern are listed below and to date the structures proposed have not met all these criteria and have tended not to drain successfully nor collapse as required.
Be strong enough to support the part as it is building. Allow full drainage of the part.
Be strong enough to maintain the dimensional stability of the finished part. Give good finish to part surfaces.
Be able to give support through the hydrostatic pressures encountered during shelling.
Be weak enough to collapse during the autoclave process.